1. Field of the Invention
This invention relates to optical telescopes and in particular to phased array optical telescopes.
2. Description of the Prior Art
The resolution requirements of advanced astronomical and surveillance telescopes indicate effective apertures of great size, beyond the range considered feasible for conventional or monolithic telescope configurations. Two alternative configurations to monolithic telescopes are available, segmented mirror configuration telescopes and phased array telescopes.
Conventional telescopes include a series of lenses or mirrors mounted along a common optical axis to form an image of a distant object. The first lens or first mirror, called the primary mirror, in the optical path is normally the largest and usually controls the light gathering power as well as the limiting resolution of the telescope. The limiting resolution may be defined as the smallest separation between two point objects which the telescope is capable of discerning.
In such conventional telescopes, all lenses and mirrors are usually manufactured from a single block of glass. These optical components are therefore termed "monolithic". The construction of monolithic optical telescopes with diameters as large as 10 meters is not currently considered feasible due to the large mass of the monolithic components and the difficulty of polishing such large surfaces to the required accuracies.
One approach developed to provide large effective apertures without the requirement of monolithic optical components of the same size is the segmented mirror, such as the 10 meter Keck telescope currently under construction for the University of California. The Keck telescope will employ a segmented primary mirror. Electronic sensors will detect misalignments among the mirror segments which will then be used to correct or minimize such misalignments. One disadvantage of the segmented mirror approach is that many segments must be constructed which, if the overall mirror profile is non-spherical, must include complicated off-axis portions of aspheric surfaces. One advantage of the segmented mirror approach is that each light ray entering the telescope system is reflected the same number of times it would be reflected in a conventional telescope. The segmented telescope approach incurs no additional reflection losses than would occur in a comparable monolithic telescope.
Another alternative to large monolithic telescope systems is the phased array telescope configuration. Phased array telescopes include arrays of afocal subtelescopes arranged about a beam combining telescope which combines the subtelescope beams into a single image. All optical components are on-axis, and are thus easier to manufacture and test than the off-axis sequence of segmented mirror telescopes.
The afocal subtelescopes in a phased array telescope system have infinite focal lengths. They convert an incoming parallel, or collimated, bundle of rays into a collimated output bundle with a smaller diameter. The optical system design goal in a phased array telescope is to provide that the bundles of rays exiting the subtelescopes and entering the beam combining telescope act as a single, continuous wavefront. This requirement must hold true for ray bundles entering the subtelescopes over a range of input angles, or over the entire field of view.
In this manner, a continuous input wavefront is mapped by the array of subtelescopes into a continuous output wavefront having smaller overall diameters and larger field angles. The beam combining telescope then uses conventional imaging principles to form a final, global image from a combination of the output wavefronts from all subtelescopes. The optical path lengths for all paths through the subtelescopes must be the same for all points on the wavefronts over the entire field of view for the global image from the beam combining telescope to be an accurate, high resolution image of the original field of view of the group of subtelescopes.
A number of geometric and imaging preconditions are imposed upon the configuration and optical design of a conventional phased array telescope. The conditions will be described below in greater detail with respect to the hypothetical two-aperture phased array telescope shown in FIG. 1. The major design goal is to provide optical phasing at all field angles. To this end, the so-called "golden rule of separated telescopes" requires that the array of exit pupils formed by the subtelescopes be an exact, demagnified replica of the entrance pupil array.
Mathematical expression of this requirement is provided below with reference to FIG. 1. These requirements are conventionally interpreted to require that subtelescope linear magnification be equal to subtelescope angular magnification and that, as compared to the entrance pupils, the subtelescope exit pupils may not be rotated relative to each other. The array of exit pupils, as a whole, may be rotated relative to the entrance pupils, but relative to each other, the exit pupils must have the same orientation as the entrance pupils. This relationship is described below in greater detail with respect to FIG. 2.
Further requirements of the subaperture array configuration are related to the overall resolution of a phased array telescope. This resolution is affected by the shape of the intensity distribution on the final image plane for an object in the field of view. The exit pupil array obtained from the subtelescopes are combined in the beam combining telescope at a final image plane which may include optical, photographic, or other appropriate sensors.
The resolution of a phased array telescope may be evaluated in terms of the shape of the intensity distribution on the final image plane obtained in the case of a hypothetical point object in the field of view of the phased array telescope. It is well known that the resultant intensity distribution, known as the point spread function or PSF, is proportional to the squared modulus of the Fourier transform of the entrance pupil. For a monolithic entrance pupil, the Fourier transform would be the zeroth order Bessel function of the first kind. The narrower the PSF, the higher the resolution of the optical system.
The resolution of an optical system may also be expressed in terms of the modulation transfer function, or MTF, of the optical system. The MTF is the Fourier transform of the PSF. This is the same as the autooorrelation of the entrance pupil. The MTF indicates the contrast to be expected in the image of an object having a specified spatial frequency.
The highest spatial frequency transmitted by an optical system, such as a phased array telescope, is called the cutoff frequency and is related to the entrance pupil diameter and the operating wavelength. The design requirements for phased array telescopes related to system aberrations, and the relationship between subaperture diameters and their separation, are quite complex and must be carefully followed to prevent areas of substantially degraded resolution at spatial frequencies below the cutoff frequency. Furthermore, the beam combining telescope must transmit the correct array of exit pupils to the first image plane without further obscuration. Thus, while the spaces between subapertures will cause degradation of resolution with respect to a monolithic telescope of the same effective aperture, the maximum phased array cutoff frequency is still potentially realizable in a properly configured phased array telescope.
The requirement that the linear and angular magnification of subtelescopes be equal is conventionally interpreted to require that these subtelescopes be free of the aberration known as distortion. In conventional optical design, freedom from distortion conventionally requires that the tangent of the output field angle be equal to the product of the tangent of the input field angle and the subtelescope magnification.
It has been proposed by certain investigators that, for phased array optical telescopes, optical phasing is assured if the sine of the output field angle is equal to the product of the sine of the input field angle and the subtelescope magnification. For small field angles, however, the conventional approximation used is that the sine of an angle is approximately the same as the tangent of that angle which is equal to that angle expressed in radians. Therefore, for phased array telescopes, optical phasing has been expected to require that for each subtelescope, the output field angle be equal to the product of the input field angle and the subtelescope magnification. See for example, W. A. Traub, "Combining beams from separated telescopes," Appl Opt. 25, 528 (1986).
Such design requirements and limitations of phased array telescopes have substantially retarded the development of these systems. Designs for several such phased array telescopes have been reported in the literature. But known phased array optical telescope designs do not provide simultaneous optical phasing over a wide field of view for operations over the full visible to infrared spectral range.
What is needed is a better understanding of the actual requirements for simultaneous optical phasing in phased array telescope designs and specific designs for such systems which actually provide simultaneous optical phasing over the full visible to infrared spectral range.